(1) Field of the Invention
The present invention relates to a method for recovery of solvent which has been used in the production of a polyarylene sulfide. More particularly, it relates to a method according to which a solvent which has been used in the production of a polyarylene sulfide can be efficiently and stably recovered.
(2) Description of the Related Art
In general, solvents used for the production of a polyarylene sulfide, such as N-methylpyrrolidone, are expensive.
Therefore, in order to reduce production cost, it is necessary to recover the solvent used for the production of a polyarylene sulfide from a mixed liquid, mainly composed of the solvent, and generated in respective steps in the production of a polyarylene sulfide.
The mixed liquid, mainly composed of solvents, which is generated in respective steps of the production of a polyarylene sulfide includes, for example, (1) the solvent solution generated at a step of separating a polymer and a precipitate from a reaction mixture after the polymerization reaction of polyarylene sulfide, (2) a washing liquid generated at the step of washing the polymerized polyarylene sulfide, or (3) a mixture of these solvent solutions and washing liquid.
Such mixed liquid (mainly composed of a solvent used for the production of a polyarylene sulfide) normally contains impurities, such as unreacted monomers, oligomers, solvent deterioration products, and, in some case, polymerization aids, in addition to the solvent.
Therefore, in order to recover the solvent used in the production of a polyarylene sulfide from this mixed liquid, it is necessary to remove the above impurities from the mixed liquid.
For recovering the solvent used in the production of a polyarylene sulfide, by removing the impurities from the mixed liquid, a method has been proposed which comprises subjecting the reaction mixture, after the polymerization reaction of polyarylene sulfide, to adiabatic flashing at a high temperature condition to volatilize the solvent (Japanese Patent Kokai No. 61-53324 and U.S. Pat. No. 3,941,664).
However, the above method suffers from the problems that (1) the flashing tank requires special means, such as a specific stirring means, evaporation means, heat source, and means for feeding solids such as polymerization aids and thus the recovery cost increases, and thus the solvent used in the production of polyarylene sulfide cannot be efficiently recovered further (2) much of unvolatilized solvent used for the production of polyarylene sulfide remains unfleshed and hence the solvent cannot be recovered in high yield.
Another method has been proposed which comprises rectifying the mixed liquid by a rectification column to obtain a rectified solvent, used in production of a polyarylene sulfide, and a bottoms fractions (residue) of the rectification column, and subjecting the bottoms to evaporation to further recover the solvent remaining in the bottoms, thereby to increase recovery rate (Japanese Patent Kokai No. 62-253,624).
However, this method also has the following problems: (1) Generally, the mixed liquid contains solid matters, such as oligomers and polymerization aids, which are soluble in the solvent, and they are apt to cause blocking of the rectification column and so the solvent cannot be stably recovered. Especially when the mixed liquid contains a polymerization aid, normally solid matters are contained in a larger amount in the mixed liquid and hence the phenomenon of preventing stable recovery is more conspicuous. (2) The composition of the mixed liquid is changed, operating conditions of the rectification column vary and correction thereof takes time, and thus the solvent cannot be recovered continually and efficiently.